Mailtrack.io - The best email tracking solution I've found. Free

Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates

ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.

Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.

Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.